This invention relates to oriented polymeric films of ultra-low density, e.g., from about 0.3 to about 0.5 gm/cm.sup.3. More particularly, the invention relates to a biaxially oriented film which exhibits opacity and reduced density by the incorporation of particle-containing voids and vacant cells therein.
In the process for producing the ultramicrocellular filaments and sheets of U.S. Pat. Nos. 3,227,664 and 3,227,784, a confined mixture of a polymer plus at least one activating liquid is heated to a temperature and pressure at which a homogeneous solution is formed, and which temperature is greater than the normal boiling point of the liquid. This solution, either under autogenous pressure or higher pressure, is extruded abruptly to a region of substantially lower pressure and temperature under such conditions that a very large number of bubble nuclei exist at the extrusion orifice. The initial concentration is chosen such that vaporization of the activating liquid rapidly cools the solution to the temperature at which the polymer precipitates, and freezes in the polymer orientation produced in the rapid extrusion and expansion process. When super-autogenous pressure on the solution prior to extrusion is required to achieve adequate nucleation, the latter is obtained by dissolving a lower boiling additive in the solution which assist nucleation by increasing the internal pressure and lowering the surface tension of the solution. Although any soluble low boiling material is said to be suitable, the preferred materials are described as those which are super-critical at temperatures above the polymer melting point. Useful nucleation additives recited in these patents include N.sub.2, CO.sub.2, He, H.sub.2, methane, ethane, propane, ethylene, propylene, certain fluorinated and/or chlorinated methanes and ethanes, and equivalents. Suitable activating liquids for use in the process of U.S. Pat. Nos. 3,227,664 and 3,227,784 should preferably have the following characteristics: (a) the liquid should have a boiling point at least 25.degree. C., and preferably at least 60.degree. C., below the melting point of the polymer used; (b) the liquid should be substantially unreactive with the polymer during mixing and extrusion; (c) the liquid should be a solvent for the polymer under the conditions of temperature, concentration and pressure employed; (d) the liquid should dissolve less than 1% of high polymeric material at or below its boiling point; and, (e) the liquid should form a solution which will undergo rapid vaporization upon extrusion, forming a non-gel polymer phase (i.e., a polymer phase containing insufficient residual liquid to plasticize the structure). Useful activating liquids are said to include methylene chloride, ethyl chloride, fluorotrichloromethane, pentane, butane, and ethanol. The polymers which are said to be suitable for use in the foregoing process are members of the class of film-forming, synthetic, crystallizable organic polymers which includes polyhydrocarbons such as linear polyethylene, stereoregular polypropylene or polystyrene; polyethers such as polyformaldehyde, vinyl polymers such as polyvinylidene fluoride; polyamides both aliphatic and aromatic, such as polyhexamethylene adipamide and polymetaphenylene isophthalamide; polyurethanes, both aliphatic and aromatic, such as the polymer from ethylene bischloroformate and ethylene diamine; polyesters such as polyhydroxypivalic acid and polyethylene terephthalate; copolymers such as polyethylene terephthalate-isophthalate, and equivalents.
U.S. Pat. No. 3,250,731 describes a process for producing a low density stereoregular (i.e., isotactic) polypropylene foam sheet, tube, coating, etc., by blending the polypropylene with a blowing agent, e.g., a solvent blowing agent such as monochlorotrifluoromethane, monochlorodifluoromethane, dichlorotetrafluoroethylene, trichloroethylene, chloroform, carbon tetrachloride, and low boiling hydrocarbons such as butane, pentane, hexane, etc., in an extruder in which the temperature in the vicinity of the die face is at least as high as the softening temperature of the blend.
U.S. Pat. No. 3,787,543 describes a process for providing a foamed sheet of crystalline polymer, e.g., of linear polyethylene, isotactic polypropylene, and the like, which is similar to the process of U.S. Pat. Nos. 3,227,664 and 3,227,784, supra, but by careful control of the extrusion conditions, including omitting any deliberately added nucleating agents (e.g., finely divided pigments, delustering agents, occluded gases, etc.), regulating temperature and pressure conditions and the extrusion orifice, is able to provide a foamed product having relatively large cells.
In the process for preparing a closed-cellular foam product disclosed in U.S. Pat. No. 3,808,300, a crystalline, substantially linear polymer, e.g., polyethylene, polypropylene, and the like, is heated and melted in an extruder supplied with a pressurized foaming agent selected from the group consisting of hydrocarbons, ethers, ketones, esters and chlorinated hydrocarbons which are normally gaseous or liquid and boiling at temperatures ranging from 45.degree. C. to 70.degree. C., and a nucleating agent selected from the group consisting of (a) a mixture of (i) an acidic alkali metal salt of citric acid and (ii) a carbonate or bicarbonate which is capable of reacting with said acidic alkali metal salt at the temperature of the polymer in the extruder; and (b) a mixture of (a) and (iii) a monoglyceride of an aliphatic monocarboxylic acid of 8-20 carbon atoms; the blend of polymer, foaming agent and nucleating agent is cooled to a temperature which is lower than the melting point of the polymer by at least 2.degree. C. but not lower than the temperature at which -dP/dT becomes 20 (kg/cm.sup.2.degree. C), in -dP/dT, T being the temperature (.degree.C.) of the blend at the extruding die of the extruder, and P being the extruding pressure (kg/cm.sup.2) of the blend at the said die, and thereafter the blend is extruded from the extruder into a low pressure zone.
The polyolefin blend film-foaming process described in U.S. Pat. No. 4,055,695 employs a first blowing agent which is completely miscible with the molten blend and a second blowing agent which has a solubility of at least 0.01% by weight in the blend and has a critical temperature below the temperature of the homogeneous mixture of the molten blend and the blowing agent system at the point when the pressure on the mixture is released during extrusion. Examples of liquids which may be used as the first blowing agent include saturated hydrocarbons such as pentane, hexane, heptane, octane, methyl pentane and dimethyl pentane, unsaturated hydrocarbons such as pentene, 4-methylpentene, hexene, petroleum ether fractions, and halogenated hydrocarbons such as carbon tetrachloride, chloroform, ethylene dichloride, methylene chloride, or 1,1,3-trichloro-1,2,2-trifluoroethane. Useful second blowing agents include carbon dioxide, nitrogen, air, methane, ethane, propane, ethylene, propylene, hydrogen, helium, argon and halogenated derivatives of methane and ethane, e.g., tetrafluorochloroethane.
U.S. Pat. No. 4,377,616 describes an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer core layer possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer. The unique structure of the core layer imparts a much higher degree of opaqueness, possibly due to the effects of light scattering, than that possible by the use of opaquing pigment alone.
None of the foregoing prior U.S. patents disclose or suggest a foamed, opaque, oriented polymeric film featuring a combination of particle-containing voids as in U.S. Pat. No. 4,377,616 and vacant cells resulting from the use of a foaming agent.